Installations of the above type in their simplest form regulate the cabin temperature by comparing the demand temperature which is set by the occupant with the measured value of the prevailing cabin temperature, and then deduce from this comparison what adjustments are necessary in the cold and hot loops of the installation. The adjustments required are then optimised in the case where the installation includes an outside temperature sensor. In order to function in this way, therefore, such an installation requires at least two sensors.
In order to improve the performance of these installations, and the safety of the occupants of the vehicle while the installation is in operation, designers working in this field have tended to resort to more complex installations, which employ sensors adapted for measuring the evaporator temperature and/or the engine coolant temperature (which is generally referred to in this Application, for simplicity, as the water temperature). Some of these installations are also equipped with a temperature sensor for sensing the temperature of the blown air which is distributed from the installation into the cabin, and also, sometimes, a unidirectional or multi-directional sensor for measuring the degree of exposure to sunlight.
This provision of plural sensors adds considerably to the cost and complexity of the installation, and also increases the probability of malfunctioning or failure of the installation.